Coupling agents are commonly used in polymers prepared by anionic polymerization. Examples of suitable multifunctional coupling agents include silicon halides, polyepoxides, polyisocyanates, polyketones, polyanhydrides, dicarboxylic acid esters. Suitable polyalkenyl coupling agents may be aliphatic, aromatic or heterocyclic. Examples of aliphatic polyalkenyl coupling agents include the polyvinyl and polyalkyl acetylenes, diacetylenes, phosphates and phosphites, dimethacrylates such as ethylene dimethacrylate, and the like. Examples of suitable heterocyclic polyalkenyl coupling agents include divinyl pyridine, divinyl thiophene, and the like. Examples of suitable aromatic alkenyl coupling agents include polyvinyl benzene, polyvinyl toluene, polyvinyl xylene, polyvinyl anthracene, polyvinyl naphthalene, divinyl durene and the like. Suitable polyvinyl groups include divinyl, trivinyl and tetravinyl. Divinylbenzene (DVB) is the preferred coupling agent in U.S. Pat. No. 5,393,787, and may include o-divinylbenzene, m-divinylbenzene, p-divinylbenzene, and mixtures of the same.
Divinylbenzene is therefore a known polyalkenyl coupling agent. The coupling agent is used for the preparation of conjugated diene polymers, i.e., styrenic block copolymers as well as polymers of conjugated dienes. Divinylbenzene is highly reactive. Unfortunately, control of the reaction is difficult. This may lead to undesirable side reactions, gelation, occurrence of weak parts in the polymer structure that degrades in a subsequent reaction (e.g., hydrogenation) or upon use (oxidative stability). Moreover, DVB is only available as relatively impure monomer. It is commercial available as technical grade 80% or 55%. Some suppliers have 85% divinylbenzene for sale. The content of impurities in DVB is therefore high. As a consequence, polymers coupled with DVB as polyalkenyl coupling agent will contain impurities, which adversely affects the properties of the polymers so produced or its application, e.g., in food-contact and medical applications.
For this reason, there is a demand for a substitute for DVB in the production of conjugated diene polymers. A suitable coupling agent, however, not only needs to be pure, but it also needs to be sufficiently reactive to couple at least two living polymer arms (same or different) to form a coupled polymer. Additional arms may be a benefit, e.g., where the coupling agent forms a core onto which a large number of living polymer arms are attached. Moreover, the coupling agent should also be sufficiently robust to withstand a subsequent hydrogenation step without degradation. Now a suitable coupling agent has been found that is sufficiently pure, sufficiently active, and sufficiently robust.